William Hill Casino - Will not return my deposit

I deposited £875 in my William Hill account in June 2015 to get a bonus of £175. They didn’t give me the bonus so I withdrew it. They refused to pay it back to my ewallet account (my method of deposit) and said they would send a cheque to me instead. The cheque never reached me even though they said they had sent it twice and then when I asked them to cancel the cheque and pay to my ewallet account they said the account has been closed as I have been found to be ‘in breach of our terms of use’.

How can I be in breach of their terms of use when all I did was deposit and withdraw? I have asked them to specify what term they claim I broke all they have sent me is:

o abuse of bonuses or other promotions; and/or

o using unfair external factors or influences (commonly known as cheating); and/or

I haven’t broken any of these terms, it’s very frustrating to be accused of something you haven’t done. I asked the casino to provide me with proof but they have refused. How can I defend myself when I am unsure exactly what they are accusing me of? To a large company like William Hill £875 isn’t a lot of money but to me it is.

I was first in contact with the William Hill support team (by email) and when they refused to discuss my case any further I emailed their second opinions team who were no help and have said that as they have ‘clearly explained’ their position several times they consider the matter closed.
I would appreciate any advice on the matter of how to get my money back.